Prothesis (Word Sounds)

Prothesis is a term used in phonetics and phonology to refer to the addition of a syllable or a sound (usually a vowel) to the beginning of a word (for example, especial). Adjective: prothetic. Also called intrusion or word-initial epenthesis. 

Linguist David Crystal notes that the phenomenon of prothesis is "common both in historical change . . . and in connected speech" (A Dictionary of Linguistics and Phonetics, 1997).  

The opposite of prothesis is aphesis (or aphaeresis or procope)--that is, the loss of a short unaccented vowel (or syllable) at the beginning of a word. 

The intrusion of an extra sound at the end of a word (for example, whilst) is called epithesis or paragoge. The intrusion of a sound between two consonants in the middle of a word (for example, fillum for film) is called anaptyxis or, more generally, epenthesis.

Examples and Observations

• "And it's a hard, and it's a hard, it's a hard, it's a hard,
  And it's a hard rain's a-gonna fall."
  (Bob Dylan, "A Hard Rain's A-Gonna Fall." The Freewheelin' Bob Dylan, 1962)
• "My characters will hence forth go afishing, and they will read Afield & Astream. Some of them, perhaps all of them, will be asexual."
  (E.B. White in a letter to a New Yorker editor who changed the word fresh to afresh in one of his essays)
• "[A prothetic sound is a vowel etc.] that has developed historically at the beginning of a word. E.g. the e of establish is in origin a prothetic vowel in Old French establir, from Latin stabilire."
  (P.H. Matthews, Oxford Concise Dictionary of Linguistics, 2nd ed. Oxford University Press, 2007)
• "Old fond eyes, beweep this cause again."
  (King Lear in The Tragedy of King Lear, by William Shakespeare)